1. The Field of the Invention
The present invention relates to a method for a radar for detecting interference between the radar and some other radar. The present invention further relates to an interference detecting device for a frequency modulated continuous wave (FMCW) radar and to the FMCW radar equipped with the interference detecting device using the method for detecting interference between the radar and some other radar.
2. Description of the Prior Art
A number of automotive radar systems which are suited to vehicle safety systems, for example, crash protection systems that minimize the effects of an accident, reversing warning systems that warn the driver that the vehicle is about to back into an object such as a child or another vehicle and the like, are known. Hence, it is important for these automotive radar systems to provide the driver with some information as to the nature or location of a target object. One target characteristic of great importance is the distance from the radar to the target object (the downrange distance). In particular, if there are multiple target objects, distances to those target objects are important information for the driver. Thus, it is obvious that radars that provide accurate downrange information for multiple target objects are desired.
The simplest automotive radar systems use a continuous wave (CW) radar in which a transmitter continuously transmits electromagnetic energy at a single frequency. The transmitted electromagnetic energy is reflected by a target object and received by the radar receiver. The received signal is shifted due to Doppler effect by movement of the target object relative to the radar. The CW receiver filters out any returns without a Doppler shift, i.e., targets which are not moving with respect to the radar. When the receiver detects the presence of a Doppler shifted signal, the receiver sends a notification containing information about presence of the target object.
Another type of radar is a two-frequency CW radar. The two-frequency CW radar transmits electromagnetic energy at a first frequency and a second frequency. The transmitted energy is reflected by a target object and received by a two-frequency receiver. The receiver measures the difference between the phase of the signal received at the first frequency and the phase of the signal received at the second frequency. The distance to the target object can be calculated from the measured phase difference. Unfortunately, the two-frequency CW radar performs poorly when there are multiple target objects at different ranges, and thus the range measurement obtained from a two-frequency CW radar in the presence of multiple target objects unreliable.
There have been known FMCW radars used as vehicle-mounted radars to detect the presence of target object or obstacles, distance to a preceding vehicle, and relative speed of the preceding vehicle from the vehicle equipped with the FMCW radar.
In order to detect target characteristic such as presence of a preceding vehicle, downrange distance to the preceding vehicle, and relative speed of the preceding vehicle, the FMCW radar transmits a radar wave via a directional antenna unit. The frequency of the radar wave is modulated so as to linearly vary in time. After the target object reflects the radar wave, the reflected radar wave is received by the radar and transformed into a received signal to be subjected to signal processing for obtaining the target characteristic. The FMCW radar mixes the transmission signal and the received signal to produce a beat signal. The beat signal is subjected to a frequency analysis, for example, a fast Fourier transformation (FFT) and the like, to obtain the peak frequencies of the beat signal (beat frequencies) from which the distance to the target object and the relative speed between the FMCW radar and the target object can be determined. The frequency spectrum has peak intensities in the intensity versus frequency characteristic curves. The beat frequencies have the peak intensities.
During those operations, there is a possibility that the FMCW radar receives not only the reflected wave from the target object, but also a radar wave transmitted from some other radar installed in another vehicle, such as a vehicle running on the same or other side of the road (e.g., a preceding vehicle or an oncoming vehicle). That is, interference between the FMCW radar with which the subject vehicle is equipped and the other radar installed in the other vehicle may occur. As a result of interference, it is hard to detect the beat frequencies accurately, and the distance to the target object such as the preceding vehicle or the relative speed of the target object cannot be accurately detected.
In Japanese Published Patent Application No. 2002-168947, Hirata et al. discloses an FMCW radar which provided with an interference detecting unit that determines whether or not the FMCW radar is interfered with by some other radar. The interference detecting unit of Hirata et al. determines whether or not interference between the FMCW radar equipped with the interference detecting unit and the other radar occurs based on an incident radio wave received by the FMCW radar or a beat signal which is generated from radar wave transmitted from the FMCW radar and the incident radio wave received by the FMCW radar. The incident radio wave may include not only return of the radar wave reflected from a target object, but also radio wave transmitted from the other radar or return of the radar wave from an obstacle located out of a measuring distance range (radar range) of the FMCW radar. In more detail, the interference detecting unit of Hirata et al. determines occurrence of interference, if either the amplitude of the incident radio wave or the beat signal is higher than a predetermined amplitude threshold value, or a beat frequency that is a frequency component at which an intensity peak can be found in frequency spectrum characteristic of the beat signal is higher than a predetermined frequency threshold value.
The method that is adopted by Hirata et al. and is based on comparison of the amplitude of the incident radio wave or the beat signal with the predetermined amplitude threshold value is performed is derived from the following idea: when interference between the FMCW radar and some other radar occurs, a radio wave transmitted from the other radar is superimposed on return of the radar wave reflected from a target object. Hence, the amplitude of incident radio wave which enters the FMCW radar or a beat signal which is generated by mixing a radar wave transmitted from the FMCW radar and the incident radio wave should be increased as compared with cases where no interference present.
The method that is adopted by Hirata et al. and is based on comparison of the beat frequency component with the predetermined frequency threshold value is performed is derived from the following idea: when the beat frequency that is the frequency component at which intensity peak can be found in frequency spectrum characteristic of the beat signal is higher than a predetermined frequency threshold value, the origin of the beat frequency component can be attributed to some obstacle located out of a measuring distance range (radar range) of the FMCW radar.
However, the methods of Hirata et al. may give an erroneous determination of presence of interference in the case where the amplitude of a radio wave transmitted from some other radar is low. That is, when the absolute amplitude of an incident radio wave or a beat signal is not so high to exceed a threshold value, the interference detecting unit of Hirata et al. can not detect occurrence of interference. Further, if low frequency noise is superimposed on the incident radar wave or the beat signal due to some reason, for example, short distance between a transmitting antenna through which the radar wave is radiated from the FMCW radar and a receiving antenna by which the incident radio wave is received, and the like, the absolute amplitude of the incident radio wave or the beat signal may exceed the threshold value, even if interference is absent.
In Japanese Published Patent Application No. 2006-300550 and the corresponding U.S. Pat. No. 7,187,321, Watanabe et al. discloses an FMCW radar that has an improved accuracy for detecting occurrence of interference using variations of absolute amplitude of an incident radio wave received by the FMCW radar or a beat signal which is generated from radar wave transmitted from the FMCW radar and the incident radio wave received by the FMCW radar. In the FMCW radar of Watanabe et al., the incident wave or the beat signal is sampled at a predetermined interval to generate amplitude data. The variations of the absolute amplitude of the incident radio wave or the beat signal are calculated by comparing two sampled absolute amplitude values at neighboring sampling points. If the maximum amplitude of the variations exceeds a predetermined value, it is determined interference occurs. If the predetermined interval at which the incident wave or the beat signal is sampled becomes shorter, it may be possible to detect occurrence of interference even if the amplitude of a radio wave transmitted from some other radar is low. However, shortening the predetermined interval may not be recommended because this leads to increasing the amount of computation in some situation, for example, where it is not easy to perform large scale computation by a vehicle-mounted FMCW radar.
Further, as previously discussed about the methods of Hirata et al., if low frequency noise is superimposed on the incident radar wave or the beat signal, the variations of the amplitude of the incident radio wave or the beat signal may exceed the threshold value, even if interference is absent.
Further, if the method of Watanabe et al. is combined with method for detecting direction of a target object such as the multiple signal classification (MUSIC) method in which incident radio wave entering a radar is received by a plurality of receiving antennas to generate a plurality of data signals each of data signals being generated by incident radio wave received by the corresponding one of the receiving antennas, and from the historical data of the plurality of data signals are constituted a self-correlation matrix by which the direction of the target object can be calculated, detection of occurrence of interference between the radar and some other radar is important because an erroneous detection of occurrence of interference at some moment influences on the accuracy of detection of direction of a target object over a long time subsequently.
Therefore, it is desired a radar that is capable of reducing computational cost to detect occurrence of interference between the radar and some other radar reliably, and to measure target characteristic such as presence of a target object within the measuring distance range of the radar system, distance between the radar system and the target object, and relative velocity of the target object to the radar system accurately, even if some large or long obstacles such as trucks and lorries, or large and long buildings such as a freeway bridge and its piers exist beyond the measuring distance range of the radar, and even if there are multiple target objects within the measuring distance range of the radar.